In recent years, as uses of light emitting diodes (LEDs) have become diverse, for example, as they come to be applied to headlamps and brake lamps of automobiles, traffic lights, or the like; there has been a demand for improved light output of LEDs.
LEDs are generally semiconductor light emitting diodes having a structure including a p-type semiconductor layer and an n-type semiconductor layer between a top surface electrode and a rear surface electrode, and a light emitting layer provided between the semiconductor layers. The ratio of light emitted from an LED to the total light produced in the light emitting layer is referred to as light extraction efficiency. An Electrode blocks light produced in a light emitting layer, which reduces light extraction efficiency to have been an obstacle to the improvement in the power output of LEDs.
Known techniques for reducing light blocked by an electrode to improve light extraction efficiency include the following. First, PTL 1 discloses a technique of providing a current blocking layer between an electrode layer and an ohmic contact layer, right under a bonding pad, and flowing a current at a portion other than a portion right under the pad, thereby reducing light blocked by the pad. In PTL 1, a semiconductor having an opposite conductivity to an ohmic contact layer is used as the current blocking layer instead of SiO2 or the like.
Further, PTL 2 discloses a technique of providing a light reflective layer made of Au or the like and a light transmitting layer having high thermal conductivity, made of AlN or the like, between a support substrate and a lower clad (semiconductor layer), thereby effectively reflecting light toward the rear surface, while improving dissipation of heat from a light emitting layer.
The structure disclosed by PTL 2 is favorable in terms of improving light extraction efficiency, since AlN which also serves as a light transmitting insulating film is located between the semiconductor layer and the light reflective layer, which suppresses diffusion of impurities into the light reflective layer from the semiconductor layer, and thus a highly reflective interface can be obtained.